Fix address literals not being treated as compile-time constants

The early return implemented for address literals in TypeChecker was
preventing the isPure annotation from getting applied. Closes ethereum#2441

Changelog.md

@@ -13,6 +13,7 @@ Features:
  * Code Generator: Added the Whiskers template system.
 
 Bugfixes:
+ * Type Checker: Fix address literals not being treated as compile-time constants.
  * Type Checker: Make UTF8-validation a bit more sloppy to include more valid sequences.
  * Fixed crash concerning non-callable types.
  * Unused variable warnings no longer issued for variables used inside inline assembly.

libsolidity/analysis/TypeChecker.cpp

@@ -1726,21 +1726,21 @@ void TypeChecker::endVisit(Literal const& _literal)
 	if (_literal.looksLikeAddress())
 	{
 		if (_literal.passesAddressChecksum())
-		{
 			_literal.annotation().type = make_shared<IntegerType>(0, IntegerType::Modifier::Address);
-			return;
-		}
 		else
 			m_errorReporter.warning(
 				_literal.location(),
 				"This looks like an address but has an invalid checksum. "
 				"If this is not used as an address, please prepend '00'."
 			);
 	}
-	_literal.annotation().type = Type::forLiteral(_literal);
-	_literal.annotation().isPure = true;
+	if (!_literal.annotation().type)
+		_literal.annotation().type = Type::forLiteral(_literal);
+
 	if (!_literal.annotation().type)
 		m_errorReporter.fatalTypeError(_literal.location(), "Invalid literal value.");
+
+	_literal.annotation().isPure = true;
 }
 
 bool TypeChecker::contractDependenciesAreCyclic(